Master issue for general container functionality

Combining a bunch of disparate issues here.

- [ ] Deleting a control from its parent through the control itself rather than the parent (**maybe**) (see #41 for rationale)
- [ ] Consistent behavior of NULL children, including on single-child containers
- [ ] uiGrid behavior when asking to add a cell that is already occupied (already exists) or remove one that isn't (doesn't)

Start by reviewing the three checklist items in this issue and the rationale in #41, then inspect the existing container and uiGrid behavior. The work is done when the requested deletion, NULL-child, and occupied-or-missing-cell cases have consistent, defined behavior.

Written by the indexing model from the issue text.